A wide variety of automobile components have traditionally been formed from steel alloys and, more recently, from aluminum alloys. In many instances, the corrosion resistance of these components may be bolstered by application of coatings to better ensure product longevity in corrosive environments. The coatings may be barrier coatings, intended to exclude a corrosive environment from the article, or sacrificial coatings, which, on exposure to a corrosive environment, will corrode in preference to the article. These coatings, which may include metal and organic coatings, may be applied using a variety of methods including dipping the article in molten metal baths, metal spray, painting and various electrodeposition processes such as electroplating.
In recent years, magnesium alloys, particularly as castings, have been the subject of considerable investigation and development due to their relatively low density relative to other structural metals. But, such magnesium-based alloys, generally containing 85% by weight or greater of magnesium and alloying elements such as aluminum, zinc, silicon and manganese, among others, are prone to corrode when exposed to water or to aqueous salt solutions. Galvanic corrosion is particularly troublesome when the magnesium alloy is in contact with other, less corrosion-prone alloys, such as the steel and aluminum commonly used in automobiles.
Magnesium alloys are not particularly well suited to many of the coating processes developed to protect steel and aluminum against corrosion and are incompatible with some. For example the phosphating process, intended to deposit a corrosion-resistant phosphate layer on steel will be rendered ineffective after some exposure to magnesium alloys. Hence many of these cast magnesium alloy articles have been used in environments where no exposure to corrosive media is anticipated or any such exposure may be well controlled. Typical applications have included steering column components, instrument panel beams, clutch and brake pedal brackets, transmission cases and housings, among others,
A broader range of applications for metals and alloys generally, and for magnesium alloys in particular may be anticipated if their corrosion performance may be improved, for example by application of a surface coating with a more corrosion-resisting or a sacrificial composition. But conventional coating processes have not proven particularly suitable for magnesium alloys and currently-available alternative coating technologies such as spark-anodizing (micro-arcs, plasma electrolytic oxidation, etc) or thermal spray processes have enjoyed only limited success.
What is therefore needed is an efficient and easily-operated technique to modify the surface composition of metal articles and magnesium alloy components in particular to render such components more corrosion-resistant.